1. Field of the Invention
This invention relates to processes involving gas-solid interaction, and, in particular, such processes utilized in fabricating devices.
2. Description of the Prior Art
A variety of techniques involving gas-solid interactions are employed in fabrication of devices such as semiconductor devices. For example, in preparation for a subsequent metallization, a semiconductor material often is etched to produce grooves in a desired pattern on its surface. One desirable etching procedure, discharge etching, involves subjecting a material to an electrical discharge or to species obtained through an electrical discharge. (In this context an electrical discharge is an ionized or partially ionized gas sustained by an electrical field such as that produced by the application of a D.C. potential, a radio frequency electrical field, or electromagnetic radiation.) Another etching process involves the production of etching species in a gas or at a gas-solid interface by treatment with a high intensity optical source such as a laser. In these processes an etchant resistant delineating material is generally deposited onto a device precursor, i.e., a structure being processed into a device. The delineating material is then patterned by conventional techniques such as photolithography so that a desired pattern is defined by the remaining portions of the delineating material. When the device precursor with its delineating layer is treated with the etchant, those portions of the device precursor that are not covered by the delineating material are removed. By choosing an appropriate etchant gas, for example, a gas in an electrical discharge at relatively low partial pressure levels, i.e., below 20 Torr, it is possible to keep the walls of the etched region substantially vertical. Thus, the pattern of the delineating material is faithfully reproduced in the device precursor by utilizing a process depending on gas-solid interaction.
The device precursor being treated by procedures such as discharge etching is often formed through other processes that involve gas-solid interactions. For example, chemical vapor deposition (CVD) is a common technique utilized for producing relatively uniform layers of semiconductor material, e.g., III-V or II-VI semiconductor materials. In the case of a CVD procedure, the deposition of a solid is induced on a substrate by producing gradients such as thermal or concentration gradients between the gas and the surface of the substrate. If the gas has appropriate constituents, the desired material is deposited on the substrate. For example, for the deposition of GaAs, HCl is passed over liquid Ga to produce a gas including GaCl. A mixture of this gas containing GaCl with gaseous AsH.sub.3 is flowed through a heated region that includes solid material upon which deposition is desired. Contact with the heated material induces the formation of GaAs on the surface.
Fabrication processes such as CVD and electrical discharge etching, e.g., reactive ion etching, reactive sputter etching and plasma etching, that rely on gas-solid interactions are crucial to the production of a variety of commercially significant devices. It is quite important to control these fabrication processes so that devices are produced that each have essentially uniform electrical and physical properties. In the context of a CVD process, the quality and reproducibility of a deposited semiconductor layer strongly depend on the careful control of the process parameters. Similarly, the quality of the pattern (e.g., the faithful reproduction of a desired pattern) produced by a gas etching technique also strongly depends on the careful control of process parameters.
Numerous methods have been postulated for monitoring an individual gas phase component present in a fabrication process relying on gas-solid interactions. The ultimate goal is to adjust the fabrication process conditions with the information obtained through this monitoring to yield a desirable gas composition, i.e., a suitable concentration for each gaseous component, and to yield a desired and reproducible end result. Often monitoring procedures rely on absorption or emissive spectroscopic techniques since such procedures do not generally perturb the composition of the gas phase being monitored. (In contrast, mass spectroscopic techniques have been employed in monitoring CVD and discharge etching processes. See Ban, Journal of the Electrochemical Society, 118 (9), 1473 (1971). If the sample inlet for the mass analyzer is near the material being processed, generally, it will significantly perturb the gas and thus substantially influence the procedure. See T. O. Sedgwick, Journal of Crystal Growth, 31, 264 (1975). If, however, to avoid this problem the analyzer is removed from this area, the composition of the gas when it reaches the analyzer often differs significantly from the composition of the gas being sampled.)
In the context of etching processes, light emissions from the gas, in the infrared, visible and ultraviolet region of the spectrum, have been monitored to detect the completion of the etching process by measuring the qualitative presence or the absence of a particular component. (See, for example, U.S. Pat. No. 3,664,942 issued May 23, 1972.) In one such procedure the etching of a layer produces gas phase components that are derived from materials present in the layer being etched, e.g., if InP is being etched by a chlorine containing plasma, gaseous indium chloride is produced. When the etching has progressed through the entire thickness of the layer being etched, e.g., an InP layer, a component corresponding to a material in the etched layer but not in the underlying layer, e.g., indium chloride, will no longer appear in the gas phase. Thus an indication of the complete etching through this layer is provided.
In an alternative method, the substantial absence or qualitative presence of an etchant species is monitored. For example, if a silicon layer is etched in a fluorine discharge, atomic fluorine will be essentially absent as long as there is Si available for reaction with the fluorine. When the silicon layer is etched through, the fluorine concentration becomes significantly larger since it is not being consumed by reaction with silicon. Although such qualitative techniques are desirable for end-point detection, they yield little information that is useful in controlling the quality of the process, e.g., the accuracy of the pattern replication. Generally, as previously discussed, the quality of a device produced by a given procedure depends on the relative concentration of a particular species and not merely on its presence or absence.
Monitoring schemes requiring a more sensitive measure of gas component concentration have been considered. Attempts have been made to measure the quantitative concentration of species present in the gas phase during CVD fabrication. Specifically, Raman scattering spectroscopy has been employed. (See J. E. Smith, Jr. and T. O. Sedgwick, Thin Solid Films, 40, 1 (1977). However, adequate control of a CVD process generally necessitates the simultaneous monitoring of a plurality of gas phase components, i.e., monitoring of two or mores species in a time period less than the deposition time, preferably less than a tenth of the deposition time, most preferably less than one-hundredth of the deposition time. This requirement is especially important in the monitoring of the CVD processes for compound semiconductor materials such as III-V or II-VI semiconductor materials or the ternaries or the quaternaries of these semiconductor materials. Raman spectroscopy, in theory, should allow such simultaneous monitoring. Despite this prediction, Sedgwick reported that Raman spectroscopy is ineffective because fluorescence induced by the laser excitation source of the Raman spectrometer completely masked the Raman signal. Since the induced fluorescence was so intense, it was natural to consider laser emission spectroscopy, i.e., the detection of fluorescence following the absorption of light from a laser excitation source. Attempts to employ laser induced fluorescence were also abandoned when simultaneous monitoring of a plurality of gas components was not achieved.
In etching processes, as opposed to CVD processes, the monitoring of a plurality of components is not always essential. However, in the presence of a discharge or a plurality of emitting species, monitoring the concentration of even one component poses many difficulties. The highly energetic gas itself produces an extremely high level of electromagnetic emissions. These emissions typically are not suitable as a quantitative monitoring expedient. Often the emitting species are not the ones whose concentration gives a measure of process quality. Additionally, the intensity of emissions such as discharge emissions depends not only on the concentration of emissive components but also on the availability of energetic electrons to excite these components. The extent of electron/component interactions is not controllable and varies with many process conditions. (See J. W. Coburn, M. Chen, Journal of Vacuum Science and Technology, 18, 353 (1981) and C. J. Mogab et al., Journal of Applied Physics, 49, 3796 (1978).) Since electron/component interactions vary irregularly with changes in many important process conditions, such as gas pressure or component concentration and since the dependence of electron/component interactions on process conditions is typically unknown, the emission intensity generally yields no easily discernible information concerning control of etching processes. The high energy excitation of the gas in etching processes and the possible large spatial gradients associated with high energy processes also appear to suggest difficulty in the adequate spectroscopic monitoring of etching processes. Thus, in general, adequate quantitative monitoring of the concentrations of components in processes involving gas-solid interactions is not a reality.